1. Field of the Invention
This invention relates to lithography and, more particularly, to photolithography.
2. Art Background
A variety of lithographic processes are presently used in the fabrication of semiconductor devices. In particular, a common lithographic technique is to employ light to expose an organic polymer that has been coated on a substrate such as a silicon wafer. The polymer is formulated so that the regions exposed to light undergo a chemical reaction. For positive resists, the chemical reaction results in a compound in the exposed regions that has, in an appropriate solvent, a significantly greater solubility than the original polymer. The pattern delineated in the polymer is developed by using this solvent to dissolve the exposed regions while leaving essentially unaffected the portions of the resist that have not been subjected to actinic radiation.
The resolution attained is affected by the wavelength of the exposing radiation. Generally, the shorter the wavelength of the exposing radiation, the better the resolution that is ultimately achievable. Therefore, extensive research has centered on the formulation of resists that are suitable for exposure by short wavelength ultraviolet light, i.e., by light in the wavelength range between 200 nm to 300 nm.
Generally, for a resist material to be appropriate for ultraviolet lithography, it should have a relatively high sensitivity to ultraviolet radiation, i.e., should require less than 200 millijoules per cm.sup.2 of incident short wavelength ultraviolet light to form the desired image. The requisite of high sensitivities is necessitated by practical consideration. Generally, useful sources of short wavelength ultraviolet radiation generate a relatively low intensity. Thus, to attain adequate exposure during a time period acceptable for most applications, a material with relatively high sensitivity to ultraviolet radiation should be utilized.
A variety of attempts have been made to produce materials that are patternable by exposure to ultraviolet radiation. For example, U.S. Pat. No. 3,558,311, issued Jan. 26, 1971, describes the solution degradation of a few copolymers of methyl methacrylate and 3-oximino-2-butanone methacrylate (OBM). Additionally, poly(methyl methacrylate) (PMMA) is also know to absorb in the short wavelength ultraviolet. These polymers have a non-negligible absorption coefficient in the ultraviolet region. However, the sensitivity of PMMA to short wavelength ultraviolet radiation is unacceptable and the liquid phase experiments done with the copolymers yielded no information concerning the solid phase characteristics such as sensitivity and resolution.
The absence of disclosures concerning a sensitive UV resist is indicative of the difficulty in developing such a resist. The sensitivity of a resist to a given exposing radiation depends on the change in solubility produced in the exposed region of the film through an induced chemical reaction. Although certain organic functional groups are known to absorb in the UV, the resulting chemical transformation(s) in the solid state of molecules having these groups is a complex and uncertain matter. Therefore, although generally it is determinable whether a material will absorb in the UV, it is not predictable how the compound will be altered or what the change in solubility will be as a result of this absorption. Thus, resists absorbing short wavelength ultraviolet radiation have been developed, but resists with relatively high sensitivities are typically not available.